I have some great quality (1920x1080 AVC/AAC) mp4 files that I want to add ssa files as soft subs. I do not want to re-encode the video because it's already excellent quality. Instead of spending hours converting the videos, I'd like to have some software just pass through the video as is and add in the sub as soft sub, so it should take minutes instead of hours. Is there a way to do it? I've tried several solutions but they all seem to take a long time to "convert" and I have many files.

If you're just planning to view the MP4 videos on a computer, you can leave the subtitles as-is and load them in whatever player you're using (or make sure they have the exact same name as the video, and the player may load them automatically), OR use mp4box (command-line program, though there are GUIs available, as mentioned on the VideoHelp Tools page for mp4box), and mux the SSA into the MP4 as a (switchable/soft) subtitle stream.

Are you intending to play the MP4 on a computer, or a standalone device? If you're playing the video on a computer, I'd just leave the .ssa as it is, and load it while playing the movie, than convert it to .srt.

I looked at the MyMP4box official site and saw that it supports the same subtitle formats as YAMB (srt, ttxt but not ssa). I don't mind losing formatting, I just want to have subs. By the way after muxing the converted srt with the video with YAMB the subtitles can't be seen either. Something must have went wrong.

From the time it takes, I'd say so, but at least the subs come out correctly (though there's some "stuttering" - each time when changing lines it flashes a couple of times). I have yet to find a solution that doesn't re-encode but mux ssa correctly.

(Although I used a custom setting and set all the criteria to "default" and when I checked the output video it has all the same criteria as the original file)

I'm trying to watch it on my iPad. I've used AcePlayer on iPad and it was able to play higher resolution than what Apple officially supports. So the only problem is to get the subtitles embedded into the mp4 since the iPad can't read external subtitle files. So what I've been trying to do is to have some program convert the text in the ssa file to something that can be embedded, such as srt, and then mux, but not too much success. So the next best thing is to have hard sub which takes more time but the subtitles display properly, as in Format Factory. But since I'm spending the time to hard code it anyway I might as well convert it to native iPad format while I'm at it. It looks like there's no way around it.

Thanks. I checked them out and as with other solutions, they require the subtitle files to be in srt or ttxt. The problem is I haven't found a reliable way to convert my ssa files to srt especially since they are non-English (Traditional Chinese).

And anyway hard sub is probably better for me since the formatting is kept. The only downside is time but I converted 26 episodes in one night and they are all on my iPad now.